1. Field of the Invention
This invention relates to an improved speaker coil and directly coupled push-pull amplifier circuit for a speaker.
2. Prior Art
A typical speaker arrangement includes a cone or vibrating structure which when vibrating with the frequency of the sound to be produced causes the speaker to emit that sound. In an audio system, the sound to be produced is generated by an electrical signal typically of constant or substantially constant voltage, with frequency variations equal to the frequency variations of the sound. It is the function of the speaker to convert this electrical signal into mechanical vibrations and, hence, sound.
One technique for this conversion involves the sending of an electrical signal representation of the sound to be produced through a voice coil placed in a magnetic field. It is well known that when an electric charge moves in a magnetic field a force is exerted upon that charge. By applying an electrical signal, such as the amplified signal produced from a sound recording or radio receiver, to a coil of wire within a magnetic field, the coil can be moved at a frequency corresponding to the frequency variations in the applied electrical signal. Coil movement is transformed to sound through a cone or other vibrating structure of the speaker. One problem in speaker design is to achieve the conversion of electrical energy to sound in an efficient manner. Applicant's copending application now U.S. Pat. No. 4,130,725 discloses a split voice coil directly coupled to a push-pull type circuit to achieve efficient conversion. An earlier directly coupled split voice coil used with a push-pull circuit is disclosed in U.S. Pat. No. 2,959,640 issued to J. B. Schultz. Speaker coil circuits known within the art, however, are inefficient energy converters at high frequencies. The inefficiencies result from changes in the inductive reactance of the speaker coil with the driving frequency. At high signal frequencies the windings of a voice coil act as an inductor and therefore present a high impedance to those high frequency signals. As the load resistance or reactance increases with an increase in frequency, the power delivered to the speakers will decrease, since the input signal from the amplifier or driving circuit will remain constant.